Background: Colorectal cancer (CRC) is a worldwide problem, with 1.2 million new cases annually. Dietary fiber has long been thought to play a protective role in CRC; yet the mechanisms by which fiber protects against CRC remain poorly understood. We recently found that low abundance of the human gut bacteria Clostridia is associated with increased risk of CRC. Clostridia ferments indigestible fiber to butyrate and other anti-carcinogenic short chain fatty acid metabolites. The current research gap is a lack of data linking these gut microbes and their metabolites to CRC. Hypothesis/Aims: The goal of this epidemiologic study is to identify inter-relationships of fiber intake, these gut bacteria and thei metabolites with risk of CRC. Our hypothesis is that fiber fermentation to specific short- chain fatty acid metabolites by Clostridia and other gut bacteria prevents CRC. Our specific aims are to 1) reestablish the Human Microbiome and Colorectal Tumor (HMAC) study population; 2) determine the relationship of fecal short chain fatty acid metabolites with risk of CRC; and 3) determine the relationship of Clostridia and other bacterial abundances and fiber intake with fecal short chain fatty acid metabolite levels. Approach: We will complete recruitment of 300 human subjects (150 CRC cases and 150 tumor free controls) at the NYU Langone Medical Center. Fecal biospecimens will be collected and fiber intake and demographic information will be assessed using validated questionnaires. Gut microbiome and their short chain fatty acid metabolites from fecal samples will be assayed using bacterial gene sequencing and LC/MS//GC/MS platforms, respectively, in a subset of samples (50 cases and 50 controls). Data analysis will be by multivariate logistic and linear regression, with control for confounding factors. Significance: Before Hurricane Sandy, we had recruited an initial 100 of 300 planned participants in the HMAC study, however; due to the storm we have experienced loss of staff time for human subject recruitment, loss of pilot data, and physical relocation of the PI's laboratory. This proposed project is critical in reestablishing research population infrastructure and rebuilding preliminary data damaged from Sandy. Innovation: This is the first human based study to examine fiber intake, specific gut microbiota and their metabolites in relation to CRC risk. Expected results: We expect to find that high levels of fecal short chain fatty acids are associated with decreased risk of CRC, and to find that the relationship of fiber intake and short chain fatty acid levels are stronger in high Clostridia carriers, compared to low Clostridia carriers. Impact: This study will clarify the etiologic basis of the fiber-CRC relationship and provide directly applicable information for tailored dietary guidance for gut microbiome-related CRC prevention.